1. Field of the Invention
The present invention relates to magneto-resistive effect elements (MR elements). Specifically, the present invention relates to the structure of a spacer layer.
2. Description of the Related Art
A thin film magnetic head that uses a giant magneto-resistive effect element (GMR element) as a reproducing head has been developed to correspond with the high recording density of a hard disk drive (HDD). Especially, a GMR element using a spin valve (SV) layer contributes to providing high sensitively thin film magnetic heads since a GMR element has a large resistance change ratio (MR ratio) to a sense current that flows in an element to read records written in a recording medium.
A current in plane (CIP)-GMR element in which a sense current flows in a parallel direction to a layer surface formed for the element was a main element among MR elements using a SV layer. However, recently a thin film magnetic head using a current perpendicular to the plane (CPP)-GMR element has been known for the further high recording density. In a CPP-GMR element, a sense current flows in a perpendicular direction to a layer surface formed for the element. The laminated structure of a typical CPP-GMR element is as follows: a lower electrode layer/a base layer/an antiferromagnetic layer/a reference layer/a spacer layer/a free layer/a cap layer/an upper electrode layer. In the specification, the marks A/B/C . . . means that the layers, A, B, C, and . . . are laminated in this order. As to the reference layer, the magnetization direction is fixed by exchange coupling with the antiferromagnetic layer, and it is also referred to as a magnetic pinned layer. As to the free layer, the magnetization direction is fixed to a direction that is roughly orthogonal in the magnetization direction of the reference layer when the external magnetic field is not applied. However, once the external magnetic field is applied, it rotates the magnetization direction according to the external magnetic field. Typically, a reference layer and a free layer are made of a magnetic layer, such as CoFe; and a spacer layer is made of Cu.
A shield layer is usually embedded in a reproducing head to restrict an area of a recording medium from which the reproducing head is affected with the external magnetic field. In a CPP-GMR element, an upper electrode layer and a lower electrode layer also work as an upper shield layer and a lower shield layer. As a result, the upper and lower shield layers are electrically connected to a SV layer in series. This structure is referred to as a CPP structure. A CPP structure has merits, such as improving heat radiation efficiency, and applying a higher operating current. In a CPP-GMR element, since a smaller sectional area has a larger resistance value and an increasing resistance change amount. In other words, a CPP-GMR element is suitable for a technology that is used for a narrower track width. A narrower track width contributes to increasing the track recording density (track per inch: TPI); and it is an essential technology for the further high recording density of a HDD.
However, a conventional CPP-GMR element with a spacer layer that is made of Cu has a limited increase of resistance change amount and output, which is obtained as a result of the increase of resistance change amount, because Cu has low resistance and a small resistance change amount in the nature of things.
Japanese laid-open patent application number 2003-204094 discloses that an insulating material is distributed along the interface in a spacer layer to obtain a CPP-GMR element with higher output. The spacer layer has a structure, for example, of Cu/NOL (nano oxide layer)/Cu.
Japanese laid-open patent application number 2002-208744 discloses that a “resistance adjustment layer” is provided anywhere inside a layer or at an interface of layers, such as a magnetic pinned layer, a free layer, and a spacer layer. The resistance adjustment layer is a layer that has a mixture layer of a conductive layer and an insulating layer; and an opening ratio of a pin hole of the resistance adjustment layer is 50% or lower. Because the conductive layer is thinner and narrower, a CPP-GMR element has a higher resistance; and the MR ratio is increased. A resistance adjustment layer can be obtained by oxidizing an alloy made of two or more metals, for example, AlCu, and by selectively forming an oxidized layer made of one of the metals.
But the technology mentioned above is not appropriate in view of reliability. Since an electric current is concentrated in the thinner and narrower conductive layer and current density is increased, local migration occurs.